Fluorescent probes, such as organic dyes, fluorescence proteins and quantum dots, have enabled many discoveries in modern biology by allowing molecular and cellular processes to be imaged with high spatial and temporal resolutions. Noble metals also exhibit versatile optical properties at the nanometer scales, providing exciting opportunities for the development of imaging probes and sensors.
Raman enhancement has been observed in noble metal nanostructures deposited onto substrates, but the presence of a substrate limit their potential applications to a wider range of uses, such as imaging of molecules, metabolites, and drugs in cells. Raman enhancements have also been observed from encapsulated few-atom silver clusters, but the low synthesis yield of these clusters makes their potential application to bioimaging difficult to realize.
Moreover, metal nanoparticles with diameters above the Fermi wavelength (˜1 nm) normally do not luminesce, although a very small fraction (≦3%) of the nanoparticles has been observed to exhibit fluorescence and Raman enhancements. However, it is unclear why certain nanoparticles exhibit fluorescence and Raman enhancements while majority of the other nanoparticles do not. Moreover, it is difficult to isolate this small fraction because the structural mechanisms responsible for the fluorescence and Raman activities are not well understood.